Modified cellular particle and process for the production thereof

ABSTRACT

A modified cellular particle is provided for molding cellular articles and comprises a partially foamed thermoplastic resin containing excess foaming agent which decomposes upon heating to generate a gas. The cellular particle has a mean specific gravity of 95 - 10 percent of the specific gravity of the resin composition, the polymer is cross-linked to a gel content of 20 90 percent and the particle has substantially no surface pores. The particles have a mean volume of 0.01 - 2 cc.

United States Patent Tamai et al.

[15] 3,655,542 51 Apr. 11, 1972 [54] MODIFIED CELLULAR PARTICLE ANDPROCESS FOR THE PRODUCTION THEREOF [72] Inventors: Isamu Tamai; MinoruOyama; Atsushi Osakada; Yasuo Shinohara, all of Otsu-shi, Japan [73]Assignee: Toray Industries, Inc., Tokyo, Japan [22] Filed: Mar. 5, 1969[21] Appl. No.: 804,435

[52] US. Cl ..204/l59.2, 204/159.19, 260/25 E, 260/25 N, 260/25 B,260/949 GA, 260/889,

[51 Int. Cl ..C08f 47/10, C08f 29/04 [58] Field ofSearch ..260/2.5, 2.5E, 2.5 B; 204/l59.l7, 159.2, 159.19; 264/54 [56] References Cited UNITEDSTATES PATENTS 3,098,832 7/ 1963 Pooley et a1 "260/25 E 3,294,86912/1966 Robinson ....204/l59. 17 3,298,975 1 1967 Feild et a1 ..260/2.5

Primary ExaminerSamuel H. Blech Assistant Examiner-Wilbert J. Briggs,Sr. Attomey-Paul & Paul [57] ABSTRACT 2 Claims, 7 Drawing Figures gmmgmmn 1972' 3,655,542

SHEET 1 UF 3 Fig. I

Fig. 2

Fig. 3

v ISAMU TAMAI w MINORU OYAMA ATSUSHI OSAKADA YASUO SHINOHARA PATENTEDAPR1 1 1972 SHEET 2 0F 3 Fig. 4

grwvrvto'uz, ISAM U TAMAI MINORU OYAMA ATSUSHI OSAKADA YASUO SHINOHARADECOMPOSITION RATIO OF THE FOAMING AGENT IN THE CELLULAR PARTICLE (/o)PATEN'I'EDIPR I1 I972 3.655.542

-' sum 3 or 3 loo I fl/ wag AMOUNT OF A G GENERATED BY DECOM ITION FROMIO GRAMS THE MATERIAL COMP ON (cc) Fig. 7

VENT I ISA TAMA ORU OYAMA y I OSAKADA YAS SHINOHARA ATTORNEYS MODIFIEDCELLULAR PARTICLE AND PROCESS FOR THE PRODUCTION THEREOF BACKGROUND OFTHE INVENTION lular particle which is useful for moulding to producecellular articles. The particles are composed of a thermoplastic. resin1 predominantly selected from the group consisting of polyethylene,polypropylene, polyvinyl chloride and polyamide.

2. Description of the Prior Art Several processes are known forproducing cellular particles of thermoplastic resins.

For instance, U.S. Pat. No. 2,744,291 teaches placing a cellularparticle in a metal mold and heating and foaming the same. This processis effective for producing cellular articles having almost any shape.Heretofore, this process has been used for production of polystyrenefoam and large quantities of polystyrene cellular articles have beenproduced industrially by this process. However, such process has notbeen applied to any thermoplastic resin other than polymers of thepolystyrene series, because of difficulties in foaming and moulding.

A typical configuration of a sectional area of a cellular article,produced when U.S. Pat. No. 2,744,291 is applied to a low densitypolyethylene containing a proper nuclear agent and being cross-linked,appears in FIG. 1 of the drawings herein, showing that said invention isdifficult to apply to polymers other than polystyrene, as will bediscussed in further detail hereinafter.

According to the specification of U.S. Pat. No. 2,681,321, knownpolystyrene cellular particles are impregnated with a volatilehydrocarbon. However, if such a process is applied to polyethylene,polyvinyl chloride or polyamide, good cellular articles cannot beprepared from the product by any known process.

Japanese patent application publication No. 24073/ 1967 discloses "theprocess of adding to a thermoplastic resin a volatile aliphatichydrocarbon as a foaming agent, extruding the mixture without foamingand cutting after cooling. However, in the particles thus obtained thefoaming agent tends to volatilize in storage and subsequent heating doesnot produce a good cellular article.

According to Belgian patent 697,785, polyethylene resin is mixed with afoaming agent in an extruder, the mixture is extruded and the product iscut into particles. During this period expansion is continued to producefoamed particles, which are irradiated to obtain cross-linked and foamedparticles. Foaming and moulding are carried out by placing the particlesinto a metal mold and heating the mold to fuse each particle. However,these particles have little or no foaming potential at the time ofmolding, and there is little or no expansion.of the foaming agent. Thus,the product is a cellular particle having many small hollow internalspaces. In making the molded article, suflicient fusion of the particlescannot be obtained. Accordingly, in this Belgian patent it is consideredpreferable to compress the article at the time of molding. However, itis then necessary to provide a metal mold which is rendered moveable byattaching a piston thereto, which requires special structure.

The present invention relates to modified cellular particles ofthermoplastic resins other than polystyrene, particularly to athermoplastic resin selected from the group consisting of polyethylene,polypropylene, polyvinyl chloride and polyamide, and to a process forthe production thereof. According to the present invention, modifiedcellular particles are readily produced for. the first time and caneasily be used to produce high-quality cellular articles.

SUMMARY or THE INVENTION It has been found that cellular articles havingexcellent shock absorption properties can be produced easily frommodified cellular particles consisting of a partially foamedthermoplastic resin selected from the group consisting of polyethylene,polypropylene, polyvinyl chloride and polyamide and containing residualexcess foaming agent which decomposes upon heating to generate a gas,such particles having sufficient reserve foaming agent to provide aparticle having a mean specific gravity of 95-10 percent of the specificgravity of the polymer composition itself. The polymer is cross-linkedto a gel content of -90 percent. The particles have a mean volume of0.01-2 cc, and substantially no surface pores.

' The particles according to this invention are ideally constituted forproducing a cellular article which is suitable for many uses, forexample, for use as a shock absorber having any desired shape. Forinstance, the shock absorber may be located in the comers in the packingbox for a television set, radio or the like. It has a shock absorptionproperty which is better than that of the hitherto employed polystyrenefoam. lt

' can be made thin and the packing can be made compact. Also,

the product of this invention can be moulded into safety cushions ofvarious shapes for the interior of an automobile. It has good shockabsorption capacity and a high safety factor. Also the cellular articleis useful in in forming floats of various sizes and shapes.

An object of this invention is to provide modified cellular particleshaving substantially no surface pores for moulding cellular articleshaving excellent shock absorption property from a polymer predominantlyconsisting of either polyethylene, polypropylene, polyvinyl chloride orpolyamide.

Another object of the invention is to provide an industriallyadvantageous process for producing the modified cellular particles.

A further object of the invention is to prevent a foaming agent fromescaping from the inside of modified cellular particles in storage,which occurs when a volatile foaming agent is used to facilitate laterfoaming and moulding.

BRIEF EXPLANATION OF THE DRAWINGS FIGS. 1-3 and FIG. 6 are photographsshowing sectional areas of cellular articles, and

FIGS. 4-5 are enlarged photographs showing parts of sectional areas ofcellular particle, as will be described in detail hereinafter;

FIG. 7 shows a preferable range of decomposition ratio of a foamingagent in cellular particles according to this invention.

DETAILED EXPLANATION OF THE DRAWINGS FIG. 1 is a photograph (0.7x) of amolded cellular article with cross-section produced from cellularparticles obtained when the process of U.S. Pat. No. 2,744,291 isapplied to a cross-linked low density polyethylene containing a suitablenuclear agent. It shows that the cellular article is collapsed.

FIGS. 2 and 3 are photographs (0.7x) of a molded article withcross-sections produced from cellular particles according to the processof Belgian patent 697,785. FIG. 2 shows heat fused cellular particles,having many hollow spaces inside; heat fusing of the particles in thecentral portion of the article is insufficient. FIG. 3 shows a moldedarticle produced from cellular particles according to the Belgianpatent, but the article was steam heated in order to make heat fusing ofthe particles complete. The photograph shows how the molded particlecollapses, and illustrates that moulding while compressing (using amovable piston or the like) is essentially indispensable in this type ofprocess.

FIG. 4 and FIG. 5 are enlarged microscopic photographs of thin layer ofabout 0.05 mm. thickness obtained by slicing the particle of the presentinvention.

FIG. 4 is an enlarged photograph (22X) of a cellular particle obtainedwhen the procedure of Example I of this specification is followed.

FIG. 5 is an enlarged photograph (66X) of a cellular particle obtainedwhen the procedure of Example 11 of this specification is followed. Aswill be apparent from these photographs, the cellular particles havesubstantially no surface pores.

FIG. 6 is a photograph (0.7X) of a sectional area of a molded articleproduced from the cellular particles of the present invention. It showsthat the article is free from collapse.

DESCRIPTION OF PREFERRED EMBODIMENTS The thermoplastic resin inaccordance with this invention may be any one selected from the groupconsisting of polyethylene, polypropylene, polyvinyl chloride and polyamide, or any resin composition containing at least 50 percent of onetype or at least two types of resin selected from the group consistingof a high density polyethylene (specific gravity: 0941-0971), a mediumdensity polyethylene (specific gravity: 0926-0940), a low densitypolyethylene (specific gravity: O.9l4-0.925 measured in accordance withASTM-D792-C50 or D1505), an isotactic polypropylene,2

polyvinyl chloride and polyamide, or a copolymer composition containingat least 50 percent of the ethylene component, propylene component,vinyl chloride component or polyamide component. Any of theaforementioned resins may be combined with 50 percent or less of anothermaterial such as rubber organic or inorganic filler, stabilizer,extender or pigment. The total of these ingredients is referred toherein as the polymer composition. And the total of the polymercomposition and ingredients described hereinafter, for example, foamingagent, decomposition promoter, cross-linking accelerator, etc. isreferred to herein as the material composition."

The expression polyamide" as referred to herein is a generic namecovering various industrially produced homopolymers such as nylon 6(polycaprolactam), nylon 66" (polyhexamethylene adipamide), nylon 610(polydecamethylene adipamide), nylon l I (polyundecanamide) and nylon 12(polydodecanamide) and copolymers of these components; polymer blends ofthese are included.

The foaming agent which decomposes upon heating to generate a gas, asused in the present invention, is a so-called known decomposition-typeorganic foaming agent, for instance, diazoamino benzene,azodicarbonamide, azodicarboxylic acid ester, barium salt ofazodicarboxylic acid, strontium salt of azodicarboxylic acid,hydrazodicarbonamide, azobisisobutyronitrile, dinitrosopentamethylenetetramine, trinitrosotrimethylenetriamine, N, N-dinitroso-N, N'-dimethyl terephthalamide, benzensulfonylhydrazide, ptoluenesulfonylhydrazide, toluene-2,4-disulfonylhydrazide,ptoluenesulfonylsemicarbazide, p, p'-oxybisbenzene-sulfonylhydrazide,p,p-oxybisbenzenesulfonylcarbazide, bisbenzenesulfonylhydrazide,diphenylsulfone-3, 3-disulfonyl-hydrazide, benzene-l,3-disulfonylhydrazide, ptoluenesulfonyl-hydrazone,p,p-diphenyldisulfonylazide, ptoluenesulfonylazide, nitroguanidine,trihydrazino-symtriazine and nitrourea. However, other equivalentfoaming agents are suitable. Most preferable is a foaming agentcomposition predominantly consisting of azodicarbonamide ordinitrosopentamethylene tetramine.

The decomposition-type chemical foaming agents are added in an amount ofabout l-25 percent by weight, based on the weight of the polymercomposition. In this case, a mixture of more than 50 percent of one orboth of these two foam ing agents, with less than 50 percent of one ormore of the other foaming agents identified above, is also a preferablecomposition, all percentages herein being expressed by weight. Also, ahydrocarbon or other evaporation-type foaming agent of the hydrocarbonor halogenated hydrocarbon type may be mixed with the decomposition-typefoaming agents in an amount of less than 50 percent. These foamingagents may be added with any decomposition promoter. Said decompositionpromoter may be, for example, urea, stearic acid, citric acid, tartaricacid, and metal salts of these acids, oxide, chloride, sulfate, nitrate,carbonate and bicarbonate of lead, zinc, cadmium, chromium, iron,manganese, cobalt, calcium, barium and strontium, and others.

In the present invention, a decomposition-type foaming agent whichdecomposes upon heating at an elevated temperature to generate a gas isused as a foaming agent because it is possible to control properly thedegree of decomposition of the foaming agent by controlling theextruding composition at an extruder. When an evaporation-type foamingagent is used, which gasifies by heating, such a control is verydifficult and it is not possible in actual practice to produce particleshaving useful foaming potential.

The mean specific gravity as referred to herein is obtained by measuringthe weights of 100 cellular particles selected by random sampling, andimmersing these particles in water at room temperature and measuring theexcluded volume. For instance, mean specific gravity can be measured bythe following process:

One hundred cellular particles are selected at random and 5 their totalweight W is measured. Next, these particles are placed in a graduatedcylinder to which a predetermined amount (V of water is added. Theparticles floating above the surface of the water are pressed by meansof a flat plate to hold them below the surface of the water and thevolume V is read at the surface line. The mean specific gravity iscalculated from the following equation:

The expansion due to foaming produces a mean particle specific gravityof -10 percent of the specific gravity of the material composition.

In the process of preparing particles, only a minor part of a foamingagent added to the resin is decomposed; a major part of the foamingagent is not decomposed. This leaves foaming potential in the particle.Unless there is a sufficient foaming capacity, upon foaming and molding,hollow spaces are created among the particles, or the cellular articlemade from the particles contracts and is of inferior quality. When aparticle is produced without first partially decomposing a foamingagent, the foaming agent foams in one step upon molding; in that case agreat difference in specific gravity is obtained between the upper partand the lower part of a cellular article made from the particles, or theupper part of the cellular article does not expand to the same shape asthe shape of the metal mold. Accordingly, it is highly preferable todecompose a part of the foaming agent and to limit the mean specificgravity of the cellular particle to 95-10 percent of the specificgravity of the material composition.

The preferable decomposition ratio of a foaming agent in a cellularparticle varies depending upon the mixed amount of the foaming agentbased on the material resin. Generally, this amount is large, thepreferable decomposition ratio in the cellular particle is relativelysmall, and when the relative amount of foaming agent based on thematerial resin is small, the preferable decomposition ratio rises.However, in the present invention, it is an indispensable requirementthat the cellular particle must retain foaming potential and that thedecomposition ratio in the foaming agent must be below 70 percent.

The decomposition ratio in the cellular particle is determined by thefollowing measuring process:

A test tube is provided having an internal diameter of about 15 mm. Intothe tube about 1.5 g. of cellular particles of the present invention isplaced. The test tube is heated in an oil bath in a manner to raise thetemperature of the oil bath 2 C/minute. The total amount of gasgenerated by each existing temperature is measured and is corrected by avalue obtained by heating a reference test tube containing no foamingagent, and a graph is plotted, showing temperature versus volume ofSpecific gravity gas liberated. This is noted as V A similar test tubeis also heated in the same way, containing 1.5 g. of the materialcomposition. This volume is also plotted against temperature, and isreferred to as V,.

The relationship between the temperature and the amount of a decomposedgas is determined with respect to the material foaming agent compositionand the cellular particle of the present invention. In thetemperature-volume curves a sharp break occurs both as to V, (the amountof a gas generated from the material foaming agent composition) and V(the amount of a gas generated from the cellular particle). Reading Vand V at the point of such sharp break, the decomposition ratio(percent) is as follows:

Decomposition ratio (percent) V, V2)/ V, X 100 The preferabledecomposition ratio of a cellular particle is within the range of FIG. 7by the amount of a decomposed gas measured by the aforementioned processwith respect to a material composition obtained by adding thepredetermined foaming agent and various additives to the material resin.

Said range is an area surrounded by the equations y SO/x, y 10,000/x, y70, x 500 and x 3 when on the axis ofthe abscissa, the amount of gas(cc.) generated by decomposition from grams of the material compositionis expressed in an ordinary scale and on the axis of the ordinate, thedecomposition ratio (percent) of the foaming agent in the cellularparticle is expressed in log scale.

It is necessary for the modified cellular particle of the presentinvention to have the decomposition ratio of the foaming agentpreferably in the range of FIG. 7 and the mean specific gravity of l0-95percent of the specific gravity of the material composition.

The amount of a gas generated by decomposition from 10 grams of thematerial composition (the portion shown by the axis of abscissa in FIG.7) can be measured by the aforementioned method. Also it can bedetermined when the kind of the foaming agent, the amount of a gasgenerated from 1 g. of the foaming agent and the mixed amount of thefoaming agent in the material composition are known.

For instance, when azodicarbonamide is used as a foaming agent, (thevolume of gas resulting from its decomposition is about 200 cc./g.) andassuming parts of azodicarboxyamide are mixed with 100 parts of thematerial composition, the amount of a gas generated by decompositionfrom 10 grams of the material composition becomes 300 cc.

In order to decompose only a part of the foaming agent, it is desirableto control the temperature and residence time of the resin inside anextruder. More specifically, this is achieved by controlling thetemperatures of the barrel and the die of the extruder, and the speed ofrotation.

The decomposition temperature of a foaming agent, or of a mixture of afoaming agent with decomposition promoter, will be defined in furtherdetail hereinafter. However, when the temperature of the resin in a dieis made more than 15 higher than the defined decomposition temperature,the foaming agent decomposes almost completely and it is not possible toobtain a particle according to the present invention. The extrudingconditions for producing the particle of the present invention varydepending upon the material conditions, (type of resin, type of foamingagent and amount of foaming agent), at the time of commencing operationsunder particular material conditions; temperature of the extruder barreland the extruder die and velocity of the screw must be determined whilemeasuring the specific gravity of the particle obtained.

By this means suitable conditions of operation can be determined easilyand the particle of the present invention can be produced.

Further, it is necessary to keep the surface of the particlesubstantially free from pores, to prevent the surface from breaking toliberate a gas after extrusion. If the surface is broken and a poroussurface is formed, this indicates that decomposition of the foamingagent has become excessive and temperatures of the barrel and of the dieof the extruder must be lowered.

The gel portion as referred to in the present invention relates to theproportion of the product of a cross-linking reaction of a thermoplasticpolymer constituting a cellular particle. This is an important elementin the preparation of a highquality cellular article. The use of achemical cross-linking agent is appropriate; however, as will bementioned later, a process using ionizing rays or ultraviolet rays ismost preferable.

As to the degree of cross-linking, it is preferable to make the ratio ofthe gel portion to the polymer in the range of 20-90 percent. However,the particularly preferred range varies de pending upon the type ofthermoplastic resin. In case of a resin composition predominantlyconsisting of polyethylene, 25-60 percent of the polymer is anespecially preferable range. In the case of a resin compositionpredominantly consisting of polypropylene, 30-70 percent of the polymeris an especially preferable range. In case of a resin compositionpredominantly consisting of polyvinyl chloride, 25-50 percent of thepolymer is an especially preferable range. In case of a resincomposition predominantly consisting of polyamide, 50-90 percent of thepolymer is an especially preferable range. When the degree ofcross-linking is outside the aforementioned upper and lower limits, thecellular article is collapsed and rough and bent, and is inferior.

In case of resin compositions predominantly consisting of polyethyleneor polypropylene, a gel portion is obtained, for example, by making 0.2g. of a test sample into a thin plate and putting it in 500 cc. oftetraline, heating the mixture at 135 C for 3 hours and measuring theundissolved portion. In case of a resin composition predominantlyconsisting of polyvinyl chloride, a gel portion is determined by making0.2 g. of a test sample into a thin plate, treating it with dimethylformamide at C for 1 hour and the gel portion is determined as theamount of the undissolved part. In case of a resin compositionpredominantly consisting of polyamide, the gel portion is mea sured asfollows: One gram of a test sample is cut into small pieces whose oneside is smaller than 1 mm., and said pieces are put into 60 cc. ofm-cresol and the mixed solution is heated at 50 C for 48 hours. Theundissolved portion is the gel portion.

The condition of substantially having no porous surface as referred toin the present invention is not limited to a case wherein the interiorof the particle is porous and the surface of the particle has anon-porous skin layer, but includes a case as shown in the photograph ofFIG. 4, wherein a somewhat porous surface remains on the surface, butthe pores on the surface are small as compared with the pores within theparticle, showing an effect similar to that of substantially a skinlayer.

The mean volume of a particle is 0.01-2 cc.; it is necessary that saidvolume is within this range. When said volume is larger than that,uniform foaming is not carried out, and the central part of the particledisplays insufficient foaming. When said volume of the particle issmaller than 0.01 cc., the product does not have a small specificgravity, or a hollow space or spaces are created inside the cellulararticle.

By the present invention modified cellular particles for moldingcellular articles of resin compositions predominantly consisting ofpolyethylene, polypropylene, polyvinyl chloride and polyamide areobtained for the first time.

The advantages of the modified cellular particle of the presentinvention are as follows:

A. The modified cellular particle of the present invention ispre-expanded as soon as it is extruded after being mixed and there is nonecessity of providing a pre-expansion step as has been done heretoforein the case of polystyrene.

B. Because a resin in the softened state is cut, a particle whosesurface is smooth is obtained, which prevents gas from escaping at thetime of foaming and molding, giving good foaming properties.

C. Because the polymer is cross-linked, when the particle is foamed andmolded, a good cellular article is obtained.

The mechanical, thermal and chemical properties are enhanced. Becausecross-linking is carried out by ionizing rays or ultraviolet rays, theparticle is cross-linked after it is produced, which is veryadvantageous for freely producing a particle having a shape and sizemost convenient for a cellular article.

D. Because a decomposition-type foaming agent is used, there is nonecessity of storing a cellular particle in a sealed container as isrequired for conventional polystyrene cellular particles.

E. Because the particle of the present invention has sufficient retainedfoaming capacity, when foaming and molding are carried out in a metalmold of a fixed size, voids among particles disappear and a cellulararticle having a uniform interior is obtained. Accordingly, there is nonecessity of using a compression step for obtaining a uniform cellulararticle as in the process of Belgian patent 697,785.

The cellular particle of the present invention is produced industrially,most advantageously, by the following process.

A thermoplastic resin, especially a thermoplastic resin predominantlyconsisting of polyethylene, polypropylene, polyvinyl chloride orpolyamide is mixed with a foaming agent which decomposes upon heating atan elevated temperature to generate a gas in a convention screw-typeextruder.

Such extruder includes an elongated barrel or housing, containing anelongated worm or screw having forwardly arranged generally helicalflights, together with means for rotating the screw in relation to thebarrel, to move the resinous material along continuously from a feedarea to an outlet area. Often, the outlet includes a die through whichthe material is continuously expressed, and a continuous cutter forcutting the material into particles as it comes out of the die.

A part of the foaming agent decomposes in the extruder and the mixtureis extruded from the die with expanding, and is cut into particles bythe rotating blade of the cutter. Then, the cut particles are cooled toa temperature below the softening point of the resin. The particles areirradiated with ionizing rays or ultraviolet rays and a cellularparticle cross-linked and pre-expanded having substantially no poroussurface on the surface is produced.

Hereinbelow said process for the production will be elaborated.

Mixing of the resin and the foaming agent is carried out in thescrew-type extruder, however, the ingredients may be mixed properly inany other suitable device before being supplied to the screw-typeextruder. The extruder may be a single screw or a multi-screw extruder,either one of the vented type or the non-vented type. In short, any suchapparatus is usable if it has a screw-type transfer mechanism. Theinside of the screw-type extruder is heated (as from an external heatingjacket) so as to soften the resin and decompose a part of the foamingagent. At this time, care must be taken so that some of the foamingagent does not decompose. Accordingly, depending upon the resinmaterial, the extruding temperature varies and it is not possible todefine it generally, however, it is normally within the range of 80250C, preferably 100-20O C. Because a part of the foaming agent hasdecomposed, the resin expands as soon as it passes through the die. Andthe extruded mixture is cut into particles by the rotating blade at anytime before or after expansion but before it has cooled enough tosolidify, and it is cooled to a temperature below the softening point ofthe resin. Cutting may be effected by using any known method, however,cutting by a rotating blade is preferable, which point is an importantfeature of the present invention. When cutting is effected after coolingand solidification, a porous surface appears on the cut surface, whichis not preferable. When the extruded mixture is cut into particlesbefore cooling and solidification, and the particles are subsequentlycooled and solidified, the particles have substantially no surface poresand are highly desirable.

It is necessary for the modified cellular particle of the presentinvention to undergo a cross-linking reaction, which becomes animportant factor in producing a good cellular article. As across-linking process, a process of using a chemical cross-linking agentis a possibility. However, when a chemical cross-linking agent is used,the foaming agent tends to decompose suddenly and not smoothly at thetime of cross-linking or extrusion from a screw-type extruder, and it isaccordingly ordinarily not possible to produce a high-quality cellularparticle.

In case a cellular particle having a mean specific gravity of -95percent of the specific gravity of the material composition is produced,it is necessary to carry out extrusion by maintaining the temperature ofthe resin in the extruder die below the decomposition temperature of thefoaming agent or foaming agent composition. The cellular particleproduced by this process has a smooth skin layer, almost free from foamson the surface, as shown in the photograph identified as FIG. 5 where,as previously stated, the magnification was 66X. And virtually no gasescapes from such a smooth skin layer and the gas accordingly remainsinside the system upon foaming and a cellular article having a highfoaming magnification is obtained. A foaming agent composition as hereinreferred to means a mixture of a foaming agent and a decompositionpromoter thereof. The decomposition temperature of a foaming agent or afoaming agent composition is determined as follows:

Into a test tube having an internal diameter of about 15 mm., 0.15 g. ofa foaming agent (or this and a predetermined amount of a decompositionpromoter) and 1.5 g. of a powdered low density polyethylene are charged.The test tube is heated in an oil bath whose temperature is caused torise at a rate of 2 C/min and the amount of a gas generated thereby iscorrected by a value obtained by heating a reference test tubecontaining no foaming agent. A curve is prepared, showing therelationship between each existing temperature and the amount of the gasgenerated at that temperature. Along the curve, a break appears,corresponding to the amount of the gas generated at a point where thegas generating speed rapidly lowers. The temperature at which this breakoccurs is defined as the generating temperature.

One of the characteristics of the present invention is thatcross-linking is carried out by using ionizing rays or ultraviolet raysand a good modified cellular particle is easily obtained. Ionizing raysare, for example a ray, [3 ray, y ray, an accelerated electron beam,proton ray and neutron ray, however, industrially an acceleratedelectron beam is most easy to use.

When cross-linking is carried out, depending upon the type of resin andthe type of ray, it is often necessary to add a crosslinking acceleratorin advance. For example, when polyethylene is crossed-linked by means ofexposure to ultraviolet rays, a carbonyl compound, an azide compound orsulfur compound may be used as an accelerator, e.g., benzaldehyde,acetophenone, benzophenone, dibenzylketone, benzyl, arylenediazide,alkylenepolysulfoneazide, dibenzylsulfide and S Cl are used, but otheraccelerators may also be used. The preferred quantity of accelerator isabout 0.1-5 percent by weight based on the weight of the resin. Whenpolypropylene and polyvinyl chloride are cross-linked by means of anaccelerated electron beam, a polyfunctional substance is added inadvance, such substance having at least two radicals having activedouble bonds, for example divinylbenzene, diallylphthalate,diallylmaleate, ethylene glycol diallylate, ethylene glycoldimethacrylate, hydroquinone dimethacrylate, propylene glycoldiacrylate, propylene glycol dimethacrylate and allyl methacrylate andother equivalent compounds. Preferably, about 0. l-2O percent by weightbased on the resin is added.

When an accelerated electron beam is used to cross-link the particle to20-90 percent of said gel content, said beam may be so controlled as toplace the amount of beam absorption in the range of about 3-20mega-rads. Any temperature of irradiation may be used, but it must bebelow the softening point of the cellular particle. Normally,irradiation is carried out at room temperature.

The present invention will be further explained by reference to thefollowing examples, which are not intended to limit the scope of theinvention. All parts are by weight except where otherwise indicated.

EXAMPLE 1 To a 65 mm. extruder, 100 parts of a low density polyethylene(specific gravity: 0.92), 10 parts of azodicarbonamide (specificgravity: 1.6), 3 parts of zinc oxide (specific gravity: 5.6) and 0.3part of zinc chloride (specific gravity: 2.9) (the specific gravity ofthis material composition: 1.11 and the amount of a gas generated bydecomposition from 10 g. of this material composition: 200 cc.) weresupplied and they were mixed. The decomposition temperature of thisfoaming agent composition was about 135 C. When the barrel temperatureof the extruder was established at 150 C, almost all of the foamingagent decomposed, therefore, the barrel temperature was graduallylowered and at last the barrel temperature of the extruder was set at130 C. On the orifice plate of the extruder 27 orifices each having adiameter of 2 mm. were provided, the mixture was continuously extrudedthrough said orifices and immediately thereafter all material washot-cut continuously into pieces 2 mm. long by means of a continuouslyrotating cutter blade. The extruded mixture foamed somewhat and expandedas soon as it was extruded and the mean specific gravity of theparticles obtained after cooling was 0.48 (43 percent of the specificgravity of the material composition, decomposition ratio of the foamingagent: about 5 percent) and the particle had substantially no surfacepores. An enlarged (magnification: 22X) photograph of the sectional areaof said particle was shown in FIG. 4. Next, said particle was irradiatedwith an electron beam by a Van de Graaff electron accelerator so thatthe amount of absorbed beam was 7 mega-rads. After irradiation, when theratio of gel component of the particle was measured, it was 45 percent.

The modified cellular particle thus obtained was put into a perforatedmetal mold and steam-heated under a gauge pressure of 5 kg./cm. for 2minutes and a good cellular article was obtained having a specificgravity of 0.07, which was uniform and free from collapse. A photographof the cellular article was shown in FIG. 6.

COMPARATIVE EXAMPLE 1 (a) Following the procedure reported in Example 1,but when an electron beam was irradiated so that the absorbed beamamount was only 2.5 mega-rads, a particle having a gel content of 10percent was obtained. This cellular particle was foamed and molded underthe conditions of Example 1, however, the generated foam was collapsedand a cellular article having a specific gravity of 0.4 was obtained. Bythe same token, when the amount of absorbed beam was made 25 mega-radsand the gel content was 85 percent, a cellular article having poor foamsand a specific gravity of 0.6 only was obtained, and fusing amongparticles was not good.

COMPARATIVE EXAMPLE 1 (b) tempt to produce a specific gravity of 0.14, acellular article was barely obtained.

EXAMPLE 2 Sixty parts of a low density polyethylene (specific gravity:0.92), 40 parts of a high density polyethylene (specific gravity: 0.95),7.5 parts of azodicarbonamide (specific gravity: 1.6),

7.5 parts of dinitroso pentamethylene tetramine (specific gravity: 1.5)and 5 parts of zinc oxide (specific gravity: 2.9) (the specific gravityof this material composition being 1.09, and the amount of a gasgenerated by decomposition from 10 g. of this material composition: 290cc.) were mixed. The decomposition temperature of this foaming agentcomposition was about 175 C. After varying the extrusion temperature todetermine a preferable condition, and after settling upon 170 C in theextruder used in Example 1, the mixture was extruded and hot-cut toproduce particles having a mean specific gravity of 0.52 (48 percent ofthe specific gravity of the material composition, decomposition ratio ofthe foaming agent: about 3 percent) having substantially no surfacepores. The particles were irradiated with an electron beam byapplication of a Van de Graaff electron accelerator, so that theabsorbed beam amount was 3 mega-rads. The gel content of the particleswas 50 percent.

This modified cellular particle was foamed and moulded as in Example 1.Steam having a gauge pressure of 10 kg./cm. was used. The obtainedcellular article having a specific gravity of 0.04 was uniform and freefrom collapse and of good quality.

EXAMPLE 3 One hundred parts of a low density polyethylene (specificgravity: 0.92), 12 parts of azodicarbonamide (specific gravity: 1.6), 5parts of zinc oxide (specific gravity: 2.9) and 1 part of benzophenone(specific gravity: 1.15) (the specific gravity of this materialcomposition: 1.07 and the amount of a gas generated by decompositionfrom 10 g. of the material composition: 240 cc.) were heated. Thedecomposition temperature of this foaming agent composition was about165 C. At first the preferable condition was sought and as a preferablecondition determined to be 150 C using the extruder used in Example 1.The extruded mixture was hot-cut to obtain particles having a meanspecific gravity of 0.50 (47 percent of the specific gravity of thematerial composition, decomposition ratio of the foaming agent: about 4percent) having substantially no surface pores. This particle wasirradiated with ultraviolet ray from a distance of 5 cm. using a 400 Whigh-pressure mercury lamp for 15 minutes. As a result a particle havinga gel content of 60 percent was obtained.

EXAMPLE 4 Following the procedure of Example 1, instead of parts of alow density polyethylene the polymer material was 100 parts of anethylene, vinyl acetate copolymer copolymerized with 15 percent of vinylacetate (specific gravity: 0.94) (the specific gravity of this materialcomposition: 1.13). The decomposition temperature of the foaming agentcomposition and the extruding conditions were the same as those inExample l. The particles obtained had a (mean) specific gravity of 0.45(40 percent of the specific gravity of the material composition,decomposition ratio of the foaming agent: about 7 percent). Theparticles obtained had substantially no surface pores. When an electronbeam was irradiated into this type of particle, so that the absorbedbeam amount was 5 mega-rads, a modified cellular article having a gelcontent of 40 percent was obtained. When this modified cellular particlewas foamed and molded, a cellular article having a specific gravity of0.08 was obtained without any difficulty.

COMPARATIVE EXAMPLE 4 (a) In Example 4, when extrusion was carried outat a barrel temperature of the extruder of C, particles were obtainedwherein the foaming agent was completely unfoamed. To this particle, anelectron beam in an amount the same as that in Example 4 was irradiatedand the particle was foamed and molded under the same conditions as inExample 4. The moulded article obtained had a mean specific gravity of0.08, however, it was not uniform such that there were many hollowspaces in the upper part and the specific gravity was low in the upperpart and high in the lower part.

11 EXAMPLE Following the procedure of Example 2, instead of 40 parts ofa high density polyethylene the polymer material was 20 parts of amedium density polyethylene (specific gravity: 0.94) and 20 parts ofpolybutadiene (specific gravity: 0.91) (the specific gravity of thismaterial composition: 1.08). The decomposition temperature of thefoaming agent composition and the extruding conditions were the same asthose in Example 2. A modified cellular article having a mean specificgravity of 0.50 (46 percent of the specific gravity of the materialcomposition, the decomposition ratio of the foaming agent: 4 percent)and a gel content of 42 percent was obtained without any problemwhatsoever. The foaming and molding were also good.

EXAMPLE 6 One hundred parts of crystalline polypropylene having anisotactic degree (percent by weight of an insoluble part in boilingn-heptane) of 95 (specific gravity: 0.91), parts of azodicarbonamide(specific gravity: 1.6), 5 parts of divinyl benzene (specific gravity:0.92) and 0.3 part of a heat stabilizer (specific gravity: about 1.8)(the specific gravity of this material composition: 0.98) were heatedand mixed at 190 C. The selection of a temperature of 190 C was due tothe fact that the decomposition temperature of this foaming agent was195 C and when the extruding condition was examined, it was found that190 C was preferable using the extruder in Example l. The extrudedmixture was hot-cut to obtain particles having a mean specific gravityof 0.45 (46 percent of the specific gravity of the material composition,the decomposition ratio of the foaming agent: about 5 percent) havingsubstantially no surface pores. An electron beam was irradiated intothis particle from a Van de Graaff electron accelerator so that theamount of absorbed beam was 8 mega-rads. The gel content of the particlewas 55 percent. This modified cellular particle was foamed and molded inthe same manner as in Example In this case, instead of steam, heatingwas carried out with the use of hot air at 200 C for 4 minutes. Thecellular article obtained had a specific gravity of 0.05, and wasuniform, free from collapse and of good quality. This cellular articlewas harder than a polyethylene cellular article and free frombrittleness as possessed by a polystyrene cellular article.

COMPARATIVE EXAMPLE 6 (a) 1n Example 6 when an electron beam wasirradiated so that the absorbed beam amount became 2 mega-rads,particles having a gel content of percent were obtained. These cellularparticles were foamed and moulded under the same conditions as inExample 6, however, collapse of foams was vigorous and a warped cellulararticle having coarse foams and a specific gravity of 0.38 only wasobtained. Also, when the absorbed beam amount was made 30 mega-rads andthe gel content was made 85 percent, the product was poor, as abovedescribed.

EXAMPLE 7 A composition was prepared consisting of 100 parts of acrystalline propylene-ethylene copolymer having an isotactic degree of65 and copolymerizing 5 percent of ethylene (specific gravity: 0.91), 15parts of azodicarbonamide (specific gravity: 0.6), 3 parts of zinc oxide(specific gravity: 2.9), 3 parts of diallylphthalate (specific gravity:1.1) and 0.3 part of heat stabilizer (specific gravity: about 1.8). Thespecific gravity of this material composition was 1.05. Thedecomposition temperature of this foaming agent was 170 C. The mixturewas heated, mixed and extruded at 170 C using the extruder used inExample 1, the extruded mixture was hotcut to produce particles having amean specific gravity of 0.50 (48 percent of the specific gravity of thematerial composition). The decomposition ratio of the foaming agent wasabout 3 percent. The particles had substantially no surface pores. Tosaid particles an electron beam was irradiated from a Van de Graaffelectron accelerator so that the absorbed beam amount was 8 mega-rads.The gel content of said particle was 53 percent.

This modified cellular particle was foamed and moulded in the samemanner as in Example 1. Steam having a gauge pressure of 10 kg./cm." wasused. The cellular article obtained had a specific gravity of 0.035,being of good quality and having unifonn, fine sealed foam voids.

COMPARATIVE EXAMPLE 7 (a) 1n Example 7, when said composition wasextruded from an extruder into sheets 3 mm. thick, after cooling anelectron beam was irradiated into these sheets so that the absorbed beamamount was 8 mega-rads. Next, said sheets were cut into square shapes of3 X 3 mm. to produce cellular particles. On the surface of saidparticles porous surfaces were exposed.

These cellular particles were subjected to attempts at foaming andmolding in various ways. However, a cellular article having a specificgravity of below 0.055 could not be obtained.

EXAMPLE 8 A mixture was prepared consisting of parts of polyvinylchloride (degree of polymerization: 1,050) (specific gravity: 1.4), 50parts of dioctyl phthalate (specific gravity: 1.1), 10 parts ofazodicarbonamide (specific gravity: 1.6), 1 part of lead stearate(specific gravity: 2), 0.2 part of dibutyl tin maleate (specificgravity: 1.27) and 3 parts of ethylene glycol dimethacrylate (specificgravity: 1.06), said mixture having a specific gravity of 1.33. Thedecomposition temperature of the foaming agent in this composition was165 C. The mixture was heated and mixed at C using the extruder used inExample 1. The extruded mixture was hot-cut to obtain particles having amean specific gravity of 0.8 (60 percent of the specific gravity of thematerial composition, the decomposition ratio of the foaming agent:about 5 percent). To said particles an electron beam was irradiated froma Van de Graaff electron accelerator so that the absorbed beam amountwas 7 mega-rads. The gel content of said particle was 28 percent. Thesemodified cellular particles were foamed and moulded the same as inExample 1. Steam having a gauge pressure of 10 kg./cm. was used. Thecellular article obtained had a specific gravity of 0.06, having unifonnfoam voids and uniform elasticity.

COMPARATIVE EXAMPLE 8 (a) 1n Example 8, when to the particles obtainedby hotcuttings, an electron beam was irradiated so that the absorbedbeam amount was 2 mega-rads, the gel content of said particle was 7percent. When these particles were foamed and molded under the sameconditions as in Example 8, the foam gas escaped and a collapsedcellular article having a specific gravity of 0.75 was obtained.

EXAMPLE 9 A mixture was prepared having a specific gravity of 1.35, thedecomposition temperature of the foaming agent in this composition was Cconsisting of 100 parts of powdered polyvinyl chloride (degree ofpolymerization: 750), (specific gravity: 1.4), 30 parts of dioctylphthalate (specific gravity: 1.1), 13 parts of azodicarbonamide(specific gravity: 1.6), 3 parts of a barium-cadmium-type stabilizer(specific gravity: about 2), and 6 parts of divinyl benzene (specificgravity: 0.92). The mixture was heated, mixed and extruded at 150 Cusing the extruder used in Example 1, the extruded mixture was hot-cutto produce particles having a mean specific gravity of 0.75 (58 percentof the specific gravity of the material composition, the decompositionratio of the foaming agent: about 5 percent) having substantially nosurface pores. To said particles an electron beam was irradiated from aVan de Graaff electron accelerator so that the absorbed beam amount was9 mega-rads. The gel content of said particle was 30 percent. Thesemodified cellular particles were foamed and molded the sameas inExample 1. Steam having a gauge pressure of 8 kg./cm. was used. Thecellular article obtained had a specific gravity of 0.05, being uniformand of good quality, free from collapse.

EXAMPLE (meltingpoint 160 C) (specificgravity: '1.1), 6 parts of N,

N'-methylenebis-acry1amide (specific gravity: 0.9) and 10 partsofazodicarbonamide (specificgravity: 1.13). The mixt'urewas fed to theextruder of Example 1, and the mixture was heated, mixed and extruded ataresin temperature at a die of 170 C. By hot-cutting of the extrudedmixture, particles having a mean specific" gravity of 0.65 (58'percentof the specific gravityof thematerial composition, the decompositionratio of the foaming agent: 4 percent)were obtained. To said particlesan electron beam was irradiated from a Van de Graaff electronaccelerator'so that the absorbed beam amount was 10 mega-rads. The. gelcontent of said particle was 68 percent.

When these particles were foamed and molded the same as in Example 1(except that the steam had a gauge pressure of kg./cm. a good, uniformcellular article having a specific gravity of 0.07 free from collapsewas obtained.

EXAMPLE 1 1 *A mixture was prepared containing 100 parts of a lowdensity polyethylene (specific gravity: 0.92), 10 parts ofazodicarbonamide (specific gravity: 1.6), 3 parts of zinc oxide(specific gravity: 5.6) and 0.3 part of zinc chloride (specific gravity:2.9) (the specific gravity of the material composition:

1.1 l,the decomposition temperature of this foaming agent compositionwas 135 C) were supplied to a 65 mm. extruder and mixed at a barreltemperature of the extruder of 120 C. At that time the temperature ofthe resin inside the die was 126C. On the orifice plate of the extruder27 orifices each "having a diameter of 2 mm. wereprovided and themixture was extruded through these orifices, and immediately thereafterthe extruded mixture was hot-cut by a rotating blade into pieces about 2mm. long. The mean specific gravity of the particles obtained aftercooling was 0.80 and since the specific gravity of the materialcomposition was 1.11, the

former was about 72 percent of the latter,-and the decompositron ratioof the foaming agent was about 2 percent. The surface of'the particlewas almost completely smooth and rupture of cells on the surface couldnot be observed by the naked eye. An enlarged (magnification: 66X)photograph of the sectional area of said particle was shown in FIG. 5.To said particle an electron beam was irradiated from a Van de Graaffelectron accelerator so that the absorbed beam amount became 5mega-rads. After irradiation, when the gel content of the particlewasmeasured, it was 32 percent.

The modified cellular particle obtained was put into'aperforatedmetalmould and heated by steam havinga gauge pressureof'4 kg./cm. A goodcellulararticle having a specific gravity of 0;05, free from collapse,was obtained.

COMPARATIVE EXAMPLE 1 l (a) beam amount was made 25 mega-rads and thegel content was made85 percent, foaming was poor and an article having aspecific gravity of 0.8 was obtained wherein fusing among par- 14COMPARATIVE EXAMPLE 1 1 (b) In Example 11, the extruded mixture was nothot-cut, but cooled in a form of extrudate and withdrawn and then cut toproduce particles. To this particle an electron beam was irradiatedsimilarly to produce a cellular particle having the same gel content.When this particle was foamed and molded with the object of making acellular article having a specific gravity of 0.06, the cellular articlecollapsed. When the objective was a specific gravity of 0.08, a cellulararticle was barely obtained.

EXAMPLE l2 Eighty'parts of a low density polyethylene (specific gravity:0.92), 20 parts of a high density polyethylene (specific gravity: 0.95),10 parts of azodicarbonamide (specific gravity: 1.6), 5 parts ofdinitroso pentamethylene tetramine (specific gravity: 1.5) and 6 partsof zinc oxide (specific gravity: 2.9) (the specific gravity of thismaterial composition was 1.10, the decomposition temperature of thisfoaming agent composition was 160 C) were put into the extruder used inExample 11, heated and mixed at C (the temperature of the resin insidethe die was C), the mixture was extruded and hot-cut to produceparticles having a mean specific gravity of 0.92 (since the specificgravity of the material composition was 1.10, it was 84 percent of thelatter, the decomposition ratio of the foaming agent was about 0.5percent). The surface of this particle was smooth and rupture of cellson the surface could not be seen by the naked eye. To this particle, anelectron beam was irradiated from a Van de Graatf electron acceleratorso that the absorbed beam amount was 6 mega-rads. The gel content ofthis particle was 42 percent. This modified cellular particle was foamedand moulded the same as in Example l 1. Steam having a gauge pressure of10 kg./cm. was used. The obtained cellular article had a specificgravity of 0.038, being of good quality and free from collapse.

EXAMPLE 13 One hundred parts of a low density polyethylene (specificgravity: 0.92), 12 parts of azodicarbonamide (specific gravity: 1.6), 3parts of lead oxide (specific gravity: 2.9) and 1 part of benzophenone(specific gravity: 1.1, the specific gravity of the material compositionwas 1.08, the decomposition temperature of this foaming agentcomposition was C) were heated to 125 C inside the extruder used inExample 11 (the temperature of the resin inside the die was 130 C). Theextruded mixture was hot-cut to obtain particles having a mean specificgravity of 0.95 (since the specific gravity of the material compositionwas 1.08, it was 88 percent thereof and the decomposition ratio of thefoaming agent was about 1 percent). The surface of this particle wassmooth and rupture of cells on the surface could not be seen by thenaked eye. To this particle, ultraviolet ray was irradiated from adistance of 5 *cm. by a 400W high pressure mercury lamp for 15 minutes.

As a result a obtained.

The modified cellular particle obtained was foamed and molded by aprocess the same as that in Example 11 by heating with steam having agauge pressure of 8 kg./cm.. The cellular article obtained had a meanspecific gravity of 0.050. Its foam structure was fine, uniform andgood.

EXAMPLE 14 Following the procedure described in Example 11, instead of100 parts of a low density polyethylene, 100 parts of an ethylene-vinylacetate copolymer copolymerizing 15 percent of vinyl acetate (specificgravity: 0.94) was used. The decompositiontemperature of the foamingagent composition and the extruding conditions were the same as those inExample 11.

particle having a gel content of 56 percent was The particle obtainedhad a smooth surface, a specific gravity of 0.85 (since the specificgravity of the material composition was 1.11, it was 77 percent thereof,and the decomposition ratio of the foaming agent was 1.5 percent) wasirradiated with an electron beam so that the absorbed beam amount wasmega-rads to obtain a cellular particle having a gel content of 45percent. When this cellular particle was foamed and molded, a cellulararticle having a specific gravity of 0.06 was obtained without anyproblem.

EXAMPLE A mixture was made up of:

a. One hundred parts of crystalline polypropylene having an isotacticdegree of 95 which was measured by dropping boiling n-heptane onto thepowdered sample (which had an average particle size above 100 mesh) for24 hours, extracting the soluble part and calculating as weight percentof the remaining insoluble portion (specific gravity: 0.91).

b. Ten parts of azodicarbonamide (specific gravity: 1.6) (thedecomposition temperature of this foaming agent was 195 C),

c. Five parts of divinylbenzene (specific gravity: 0.92), and

d. 0.3 part of a heat stabilizer (specific gravity: about 1.8).

The specific gravity of the material composition was 0.96.

The mixture was heated and mixed at 180 C using the extruder of Example1 l, the extruded mixture was hot-cut to obtain particles having aspecific gravity of 0.84 (since the specific gravity of the materialcomposition was 0.96, it was 88 percent thereof, and the decompositionratio of the foaming agent was about 1 percent). The surface of thisparticle was smooth and rupture of cells on the surface could not beseen by the naked eye. To this particle, an electron beam was irradiatedfrom a Van de Graaff electron accelerator so that the absorbed beamamount was 8 mega-rads. The gel content of this particle was 50 percent.This modified cellular particle was foamed and molded the same as inExample 1 l, but in this case instead of steam, heated air at 230 C wasused to heat the particle for 4 minutes. The cellular particle obtainedhad a specific gravity of 0.047, being uniform and of good quality, freefrom collapse. Also, this cellular article was harder than apolyethylene cellular article, having no such brittleness as that of apolystyrene cellular article.

EXAMPLE 16 One hundred parts of a crystalline propylene-ethylenecopolymer copolymerizing 5 percent of ethylene having an isotacticdegree of 65 (specific gravity: 0.91), 15 parts of azodicarbonamide(specific gravity: 1.6), 3 parts of zinc oxide (specific gravity: 2.9)(the decomposition temperature of this foaming agent composition: 170C), 3 parts of diallylphthalate (specific gravity: 0.9) and 0.3 part ofa heat stabilizer (specific gravity: about 1.8), (the specific gravityof the material composition: 1.05) were heated, mixed and extruded at160 C using the extruder of Example 11 (the temperature of the resininside the die was 164 C), the extruded mixture was hot-cut to produceparticles having a mean specific gravity of 0.80 (since the specificgravity of the material composition was 1.05, it was 76 percent thereof,the decomposition ratio of the foaming agent: about 1 percent). To thisparticle an electron beam was irradiated from a Van de Graaff electronaccelerator so that the absorbed beam amount was 6 mega-rads. The gelcontent of this particle was 47 percent.

This modified cellular particle was foamed and molded the same as inExample 1 1. Steam having a gauge pressure of 10 kg./cm. was used. Thecellular article obtained had a specific gravity of 0.033, having fineclosed cell structure.

COMPARATIVE EXAMPLE 16 (a) In Example 16, said composition was extrudedfrom an extruder into sheets 3 mm. thick, and after cooling an electronbeam was irradiated to said sheets so that the absorbed beam amountbecame 8 mega-rads. Subsequently, said sheets were cut into squareshapes of 3 X 3 mm. to produce cellular particles. The mean specificgravity of this particle was 0.78. Ef-

forts were made to foam and mold these particles in various ways;however, a cellular article having a specific gravity of less than 0.046was not obtained.

EXAMPLE 17 A mixture consisting of parts of polyvinyl chloride (degreeof polymerization: 1,050) (specific gravity: 1.4), 50 parts of dioctylphthalate (specific gravity: 1.1), 10 parts of azodicarbonamide(specific gravity: 1.6), 1 part of lead stearate (specific gravity:about 1.8) (the decomposition temperature of this foaming agentcomposition in polyvinyl chloride: 165 C), 0.2 part of dibutyl tinmaleate (specific gravity: about 1.8) and 3 parts of ethylene glycoldimethacrylate (specific gravity: 0.9) (the specific gravity of thematerial composition: 1.32) was heated and mixed at 140 C using theextruder of Example 11 (the temperature of the resin inside a die: 143C), the extruded mixture was hot-cut to obtain particles having a meanspecific gravity of 1.2 (since the specific gravity of the materialcomposition was 1.32, it was 91 percent thereof, and the decompositionratio of the foaming agent: about 0.5 percent) having no recognizablecollapse of foams. To this particle an electron beam was irradiated froma Van de Graaff electron accelerator so that the absorbed beam amountwas 7 mega-rads. The gel content of the particle was 30 percent. Thismodified cellular particle was foamed and molded the same as in Example11. Steam having a gauge pressure of 10 kg./cm. was used. The cellulararticle obtained had a specific gravity of 0.055, having uniform foams,being elastic.

COMPARATIVE EXAMPLE 17 (a) In Example 17, when to the particle obtainedby hot-cutting an electron beam was irradiated so that the absorbed beamamount was 2 mega-rads, the gel content of the particle was 6 percent.When this particle was foamed and molded under conditions the same as inExample 11, the foam forming gas escaped and a collapsed cellulararticle having a specific gravity of 0.90 was obtained.

EXAMPLE 18 One hundred parts of a low density polyethylene (specificgravity: 0.92), 2 parts of azodicarbonamide (specific gravity: 1.6), 0.6part of zinc oxide (specific gravity: 5.6) and 0.06 part of zincchloride (specific gravity: 2.9) (the specific gravity of this materialcomposition: 0.93, the decomposition temperature of this foaming agentcomposition: 135 C) were supplied to the 65 mm. extruder and mixed at abarrel temperature of the extruder of C. At this time the temperature ofthe resin inside the die was 133 C. The mean specific gravity of theparticle obtained by hot-cutting was 0.55, that was 59 percent of thespecific gravity of the material composition and the decomposition ratioof the foaming agent was 22 percent. The surface of the particle wassmooth and rupture of cells on the surface could not be seen by thenaked eye. To this particle, an electron beam was irradiated from a Vande Graaff electron accelerator so that the absorbed beam amount was 7mega-rads. After irradiation, when the gel content of the particle wasmeasured it was 48 percent. When the obtained modified cellular particlewas put into a perforated metal mould and heated by steam having a gaugepressure of 5 kg./cm. for 4 minutes, a good cellular article having aspecific gravity of 0. 15, free from collapse, was obtained.

COMPARATIVE EXAMPLE 18 (a) 1n Example 18, when the temperature of thebarrel of the extruder was made 135 C, the temperature of the resininside the die became C. The particle obtained under this condition hada mean specific gravity of 0.27, however, the decomposition ratio of thefoaming agent was 75 percent. This particle was foamed and molded underthe same conditions as in Example 18, however, because it had no foamingpotential, when it was molded in an effort to obtain an article having aspecific gravity of 0.15, the cellular article collapsed. When it wasmolded in an effort to obtain an article having a specific gravity of0.2, a cellular article having no collapse was barely obtained, however,there were many hollow spaces among the particles.

The following is claimed:

1. A process for producing a modified cellular particle, which comprisesthe sequential steps of mixing in a screw-type extruder a compositioncomprising a thermoplastic resin selected from the group consisting ofpolyethylene and polypropylene and a foaming agent which decomposes upon

2. A process according to claim 1 wherein the temperature of the resininside the die is made 100*-220* C.